The frequency and severity of urinary-bladder toxicity were determined retrospectively in a large series of childhood cancer patients treated with either pelvic irradiation and simultaneous cyclophosphamide or cylclophosphamide with extrapelvic irradiation. Of 50 patients who received the first combination, 17 (34%) developed urinary-bladder toxicity. Eight of the 17 had transient hematuria and dysuria with complete clearing clinically after cessation of treatment; nine had chronic or intermittent hematuria which persisted after treatment was stopped and often resulted in demonstrable fibrosis and telangiectasia of the bladder. By contrast, of 60 children who received cyclophosphamide and radiotherapy outside the pelvic region, only five (8%) developed hematuria and in all instances it was transient. This comparative study demonstrates a significantly increased frequency and severity of urinary-bladder toxicity in cancer patients receiving pelvic irradiation with simultaneous cyclophosphamide.